The invention is in the field of transistor amplifier circuits, and relates more particularly to power amplifier circuits and power detection circuits for such power amplifier circuits.
Amplifiers of this general type are frequently used in high-frequency RF amplifiers, such as those used in wireless communications apparatus, as well as in audio amplifiers and other applications. In such amplifiers, particularly when used in RF applications, it is desirable to provide an indication of the power in the output stage.
In the prior art, this function is typically accomplished by one of two techniques. First, a directional coupler or voltage divider may be provided at the output of the power amplifier to sense a fraction of the signal applied to the load, and this sample signal is then used to indicate the power in the load. The second technique used in the prior art generates a sample of the current in the power amplifier by using a smaller transistor in parallel with the power transistor. The sample current is then fed off-chip, where it is averaged and used to provide an approximate indication of the power in the load.
However, there are various drawbacks inherent in the prior-art solutions. Thus, for example, directional couplers are bulky and lossy, resistive dividers are inefficient, and using a current sample as a measure of output power is relatively inaccurate.
Accordingly, it would be desirable to have a power detection circuit for a power amplifier circuit which is compact, efficient, easy to implement, and more accurate than prior-art power detection circuits.
It is therefore an object of the present invention to provide a power detection circuit for use in a power amplifier circuit which is compact and efficient in design, easy to implement, and which provides a more accurate indication of amplifier power than prior-art circuits.
In accordance with the invention, these objects are achieved by a new power detection circuit for use in a power amplifier circuit having an amplifying transistor, in which the power detection circuit includes a circuit for generating a signal which is directly proportional to a power level in the amplifying transistor.
In a preferred embodiment of the invention, this is done by providing a circuit for generating a voltage proportional to the square of a current in the amplifying transistor and averaging this voltage to provide an indication of this power level in the amplifying transistor.
In a further preferred embodiment of the invention, a wireless communications device having an RF power amplifier circuit is provided with a power detection circuit for generating a signal which is directly proportional to the power level in the amplifying transistor.
In further preferred embodiments of the invention, the power detection circuit may be implemented using a simple MOS transistor configuration, or a translinear circuit using bipolar transistors which is more complex than the MOS circuit but which provides greater accuracy.
A power detection circuit in accordance with the present invention offers a significant improvement in that a particularly advantageous combination of features, including a compact and efficient design, and more accurate performance, can be obtained in an economical and easily-implemented configuration.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.